Field of the Invention
The invention relates to a method for artifact-free rendering of metal parts in three-dimensionally reconstructed images of an examination object in a patient. With the aid of such a method it is possible to check screw locations, for example in spinal column surgery.
An angiography system for carrying out such a rendering method is known e.g. from U.S. Pat. No. 7,500,784 B2, which will be explained below on the basis of FIG. 1.
FIG. 1 shows a monoplanar x-ray system, depicted as an example, with a C-arm 2 held by a stand 1 in the form of a six-axis industrial or folding-arm robot, attached to the end of which there are an x-ray radiation source, for example an x-ray emitter 3 with x-ray tubes and a collimator, and an x-ray image detector 4 as an image recording unit.
The folding-arm robot, which is, for example, known from U.S. Pat. No. 7,500,784 B2, preferably has six axes of rotation and therefore six degrees of freedom. By using the folding-arm robot, it is possible to adjust the C-arm 2 in space as required, for example by virtue of it being rotated about a center of rotation between the x-ray emitter 3 and the x-ray image detector 4. The angiographic x-ray system 1 to 4 according to the invention is rotatable, in particular, about centers of rotation and axes of rotation in the C-arm plane of the x-ray image detector 4, preferably about the center point of the x-ray image detector 4 and about the center point of the axes of rotation intersecting the x-ray image detector 4.
The known folding-arm robot has a main frame, which is securely attached e.g. to a floor. A carousel is affixed thereto in a manner rotatable about a first axis of rotation. A robot rocker is attached pivotably about a second axis of rotation on the carousel, on which a robot arm is attached rotatably about a third axis of rotation. A robot hand is attached rotatably about a fourth axis of rotation at the end of the robot arm. The robot hand has an attachment element for the C-arm 2, which can be pivoted about a fifth axis of rotation and can be rotated about a sixth axis of rotation extending perpendicular thereto.
The realization of the x-ray diagnostics apparatus is not dependent on the industrial robot. It is also possible to make use of conventional C-arm devices.
The x-ray imaging detector 4 can be a rectangular or quadratic, flat semiconductor detector which is preferably made from amorphous silicon (a-Si). However, it is also possible to use integrating and possibly counting CMOS detectors.
A patient 6 to be examined as an examination object is situated on a slab 5 of a patient mounting table in the beam path of the x-ray emitter 3. Attached to the x-ray diagnostics apparatus is a system control unit 7 with an image system 8, which receives and processes image signals from the x-ray image detector 4 (operating elements, for example, have not been depicted). The x-ray images then can be inspected on displays of a monitor suspension 9. A known device 10, the function of which will still be described in more detail, is furthermore provided in the system control unit 7.
As depicted in FIG. 2 in a simplified manner, the angiographic x-ray system also can have a normal ceiling-mounted or floor-mounted holder for the C-arm 2, in place of the x-ray system with the stand 1 in the form of the six-axis industrial or folding-arm robot depicted in FIG. 1 in an exemplary manner.
In place of the C-arm 2, which is depicted in an exemplary manner, the angiographic x-ray system also can have separate ceiling-mounted and/or floor-mounted holders for the x-ray emitter 3 and the x-ray image detector 4 which, for example, are coupled in an electronically rigid manner.
The x-ray emitter 3 emits a beam 11 emanating from a beam focus of the x-ray radiation source thereof and impinging on the x-ray image detector 4. If 3D data records are to be created according to the so-called DynaCT method (a method for rotational angiography), the rotatably mounted C-arm 2 with the x-ray emitter 3 and the x-ray image detector 4 is rotated in such a way that, as shown schematically in FIG. 2 in a top view onto the axis of rotation, the x-ray emitter 3, illustrated therein figuratively by the beam focus thereof, and the x-ray image detector 4 move along an orbit 13 around an object 12 to be examined, which is situated in the beam path of the x-ray emitter 3. It is possible to pass over the orbit 13 completely or in part for the purposes of producing a 3D data record.
In this case, according to the DynaCT method, the C-arm 2 with the x-ray emitter 3 and the x-ray image detector 4 preferably moves over at least an angular range of 180°, for example 180° plus a fan angle, and records projection images from different projections in quick succession. The reconstruction can be performed while using only a portion of this recorded data.
By way of example, the object 12 to be examined can be an animal or human body, as well as a body phantom.
The x-ray emitter 3 and the x-ray image detector 4 each move around the object 12 in such a way that the x-ray emitter 3 and the x-ray image detector 4 lie opposite one another on opposite sides of the object 12.
In the case of normal radiography or fluoroscopy carried out by using such an x-ray diagnostics apparatus, medical 2D data from the x-ray image detector 4 are optionally buffer stored in the image system 8 and subsequently rendered on the monitor 9.
However, despite the application of metal artifact corrections, displaying metallic objects such as e.g. implants or screws, is still difficult in the 3D reconstructed images as a result of the occurrence of artifacts. By way of example, those artifacts can lead to misinterpretations of the screw positions, for example in the case of fusion operations on the spinal column. Such a fusion operation may be the posterior lumbar inter vertebral fusion (PLIF), a surgical technique for fusing lumbar vertebrae by removing the inter vertebral disk and replacing it by a titanium basket, wherein the vertebra is subsequently still internally stabilized by fixation, as can be gathered from e.g. the article “Posterior Lumbar Inter Body Fusion And Segmental Lumbar Lordosis” by Rahul Kakkar, et al., published in Eur J Orthop Surg Traumatol (2007), vol. 17, pages 125-129.
These days, the following methods find use for reducing the problem of the metal artifacts:                metal artifact correction,        3D scanning in the ideal rotational orientation (for minimizing the artifacts) and/or        experience-based evaluation of the artifact-afflicted images.        
German Patent Application DE 10 2011 083 063.4 proposes a method for producing planning data correlated to a placement of an implant at an operating site in a patient, in which 3D image data and, using an imaging system, 2D image data of the operation site are produced. The 3D image data are assigned, at the correct location, to a coordinate system of the imaging system using the 2D image data, a 3D model of the implant is adapted virtually into the 3D image data at the operating site and the planning data are produced in the coordinate system of the imaging system using the 3D image data containing the 3D model.